ADMIXTURES
Admixtures are added to the concrete before or during mixing, to modify one or more of the specific properties of concrete in the fresh or hardened states. IS:9103-1979 lays down the procedures for evolution of admixtures for concrete The different types of admixtures used are given below.
1. Accelerating Admixtures:
These are added to concrete to increase the rate of early strength development, which in turn facilitates earlier removal of form work. Common accelerators are calcium chloride, fluosilicates and trietanlamine.
2. Retarding Admixtures:
These are added to slow down the rate of setting of cement. They are useful in hot weather concreting. Common types of retarders are starches and cellulose products, sugar and hydroxyl-carboxylic acids
3. Water Reducing or Plasticizing Admixtures:
The addition of plasticizer allows greater workability for given water cement ratio or alternatively retains the workability while reducing the water content. The basic ingredients of water reducing agents are either lignosulphonate salts or polyhydroxy compounds
4. Air-Entraining Admixtures:
These are used to incorporate air in the form of minute bubbles in concrete usually to increase workability and resistance to freezing and thawing Commonly used air-entraining agents are animal and vegetable oils, natural wood resins and their sodium salts, alkali salts of sulphated and sulphonated organic compounds.
CONCRETE MIX PROPORTIONS
The mix proportions shall be selected to ensure the workability of fresh concrete and when concrete is hardened, it shall have the required strength, durability and surface finish.
The main objective of concrete mix proportioning is to select the optimum proportion of the various ingredients of concrete (cement, aggregate, and water), to attain the required workability (when it is fresh), strength and durability (when it is harderied) with minimum cost. It can be done in the following two ways
(a) Nominal mix concrete
(b) Design mix concrete
NOMINAL MIX CONCRETE
In nominal mix, the proportions of cement, aggregate and water are nominally adopted Nominal mix concrete is used on small works for routine concrete construction and for concrete M 20 and lower. However it requires high cement content. Nominal proportions such as 1:2:3, 1:3:6 etc. have no significance for strength and durability. In these, the quantity of fine aggregate is fixed irrespective of the cement content and the maximum size of aggregate. Hence, the variations in the quality of concrete produced are inevitable. The proportions of materials for nominal mix concrete prescribed by IS: 456-2000 are given in Table. 1.3.
DESIGN MIX CONCRETE
In design mix, the proportions of cement, aggregate, water and mineral admixtures if any are actually designed.
Over the years, several mix design methods have been developed, which are listed below
(i) Minimum voids method
(ii) Maximum density method
(iii) Fineness modulus method
(iv) Road research laboratory (RRL) method
(v) American concrete institute (ACI) method
(vi) I.S Code method
